U.S. Pat. Nos. 4,447,470 and 4,487,813 disclose state-of-the-art methods for epitaxially growing HgCdTe upon CdTe substrates. The method described in these two patents differs from that of the present invention in that the HgTe source and CdTe substrate are maintained at the same temperature during the processing step, whereas in the present invention, the HgTe source 3 is heated to a different temperature than the CdTe substrate 5 for at least part of the growth step. As a result, the compositional profile of the finished HgCdTe is desirably sharper when the present invention is used. This is graphically indicated in FIG. 2, in which the compositional profile associated with these two prior art patents is labeled "prior art" and compared with a typical compositional profile associated with the teachings of the present invention. In the prior art, the mole fraction (x) of cadmium in the epitaxially grown HgCdTe which is a distance of at least 20% of the HgCdTe layer thickness away from the CdTe substrate varies by no more than 10% as a function of said distance. The compositional profile of the present invention is sharper, in that the mole fraction (x) of cadmium in the epitaxially grown HgCdTe 15 which is a distance of at least 10% of the HgCdTe layer 15 thickness away from the CdTe substrate 5 varies by no more than 10% as a function of said distance. The present invention also offers the advantage that the x-value can be changed by changing the temperatures during the growth run, without having to change the mole fraction of Hg in the HgTe source.
U.S. Pat. No. 4,487,640 discloses a method for epitaxially growing HgCdTe onto CdTe substrates, which method differs from that of the present invention in that: (1) It uses two different sources (16 and 20) maintained at different temperatures, whereas the present invention uses one source. (2) It is not a closely-spaced process, whereas the present invention is. As used herein, "closely-spaced" means the HgTe source and CdTe substrate are spaced apart between 0.1 mm and 10 mm. (3) It uses a carrier gas (H.sub.2 +HX), whereas the present invention does not. (4) The source zones (T.sub.2 and T.sub.3) are hotter than the substrate zone (T.sub.1), which is the opposite of the temperature relationship in the present invention. (5) It does not vary the temperatures over the course of the growth phase, whereas the present invention sometimes does.
Tufte et al., "Growth and Properties of Hg.sub.1-x Cd.sub.x Te Epitaxial Layers", J. Applied Phys., Vol. 40, No. 11, pp. 4559-4568 (October 1969) describe primarily isothermal methods for growing HgCdTe onto CdTe substrates. On p. 4564 of this reference, the authors discuss their unsuccessful attempts at using a non-isothermal method. The results were unsuccessful because the grown layer had a surface that was pure HgTe rather than HgCdTe, or else had a significant compositional profile, apparently due to the interdiffusion of mercury and cadmium. The reference method differs from that of the present invention in that HgCdTe, rather the HgTe, is used as the source, and the source is kept hotter than the substrate, providing an additional driving force for the transfer of material from the source to the substrate. In the present invention, on the other hand, the substrate 5 is made to be hotter than the source 3.
U.S. Pat. No. 4,418,096 discloses a method for epitaxially growing HgCdTe onto a CdTe substrate, which differs from the method of the present invention in that: (1) It uses dangerous mercury overpressures of between 4 and 50 atmospheres, col. 4, lines 34-36, whereas the present invention does not use mercury overpressure. (2) The finished layers are at least 158 microns thick, whereas the layers 15 of the present invention are no more than 30 microns thick. (3) The processing time is at least 8 days, whereas that of the present invention is no more than 4 hours. Such a long processing time is inconvenient, to say the least. (4) The compositional profile is not nearly as good. (5) The source and substrate are always at the same temperature with respect to each other; this is not true in the present invention.
U.S. Pat. No. 3,622,405 relates to the growth of bulk HgCdTe crystals, not epitaxial layers of same, that are annealed by raising them to temperatures in the vicinity of 755.degree. C.
Vohl et al., J. Electronic Materials, Vol. 7, No. 5, pp. 659-678 (1978), discloses a "hot wall" process for making HgCdTe, rather than a closely-spaced process, in which the starting ingredients are elemental Hg, Cd, and Te.
Early investigators employing the isothermal close-spaced vapor phase epitaxy (isothermal CSVPE) growth process grew very thick layers of HgCdTe using long growth periods compared with the present invention's HgCdTe layers 15 having thicknesses of between 0.7 and 30 micrometers grown in short time periods (5 minutes to 4 hours). E.g., see Marfaing et al., "A New Process of Crystal Growth: Evaporation-Diffusion Under Isothermal Conditions", Proc. Int'l Conf. on Crystal Growth, Boston, MA, pp. 549-552 (June 20-24, 1966). Prior workers considered the growth process to be limited to a solid-state diffusion process, in which only a pseudo-control of composition is obtained, by added Hg pressure. In the non-isothermal linear growth vapor phase epitaxy (non-isothermal LGVPE) method described in the instant specification, a second mechanism, sublimation, is involved, particularly in those embodiments where support 10 is not used; this sublimation mechanism is probably the dominant growth form for the early growth period. Furthermore, the diffusivities of Hg and Cd and control of HgTe 3 and HgCdTe 15 decomposition rates are used in the present invention to produce HgCdTe layers 15 of any desired composition with compositional profiles that are far less graded than those produced by the isothermal methodology. This is because the present invention controls the source 3 and substrate 5 temperatures independently.
In the instant process, the growth rate is linearly related to time; thus, the characterization "linear growth" vapor phase epitaxy. During the growth period, and CdTe substrate 5 sublimes to provide Cd and Te.sub.2, which react with Hg to form HgCdTe 15 at the CdTe substrate 5 surface. The sublimed Cd and Te come from all surfaces of the CdTe substrate 5, but primarily from the back (i.e., the side not facing the grown HgCdTe layer 15; the top in FIG. 1). However, as the HgCdTe 15 growth process continues, this source of Cd and Te.sub.2 is attenuated due to substrate 5 being coated with HgCdTe 15 (growth on the back of the CdTe substrate 5 is much slower than on the side facing the HgCdTe layer 15). Consequently, with long growth periods, only the solid state diffusion mechanism appears to be operational.